Ensure install_venv works behind firewalls

There is a problem with setuptools and firewalls that prevents
setuptools from downloading setup_requires properly. The only thing
in openstack that really needs setup_requires is pbr. Once that's in,
everything else installs via pip, which does firewalls correctly. So,
just install the latest pbr into the venv and the rest should work.

Change-Id: I838559f6f308e4a0c255af7bc27cdc669c5cf5f3
Merge "Ensure install_venv works behind firewalls"